/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import java.util.ArrayList;
import java.util.Collection;

import sun.misc.Unsafe;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * <p>This class defines a nested {@link ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 *
 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 *  <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquires the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   public void lock()                { sync.acquire(1); }
 *   public boolean tryLock()          { return sync.tryAcquire(1); }
 *   public void unlock()              { sync.release(1); }
 *   public Condition newCondition()   { return sync.newCondition(); }
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);
 *   }
 *   public boolean tryLock(long timeout, TimeUnit unit)
 *       throws InterruptedException {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 *  <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     boolean isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }
 *   public void signal()         { sync.releaseShared(1); }
 *   public void await() throws InterruptedException {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() { }

    /**
     * Wait queue node class.
     *
     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
     * Hagersten) lock queue. CLH locks are normally used for
     * spinlocks.  We instead use them for blocking synchronizers, but
     * use the same basic tactic of holding some of the control
     * information about a thread in the predecessor of its node.  A
     * "status" field in each node keeps track of whether a thread
     * should block.  A node is signalled when its predecessor
     * releases.  Each node of the queue otherwise serves as a
     * specific-notification-style monitor holding a single waiting
     * thread. The status field does NOT control whether threads are
     * granted locks etc though.  A thread may try to acquire if it is
     * first in the queue. But being first does not guarantee success;
     * it only gives the right to contend.  So the currently released
     * contender thread may need to rewait.
     *
     * <p>To enqueue into a CLH lock, you atomically splice it in as new
     * tail. To dequeue, you just set the head field.
     * <pre>
     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * </pre>
     *
     * <p>Insertion into a CLH queue requires only a single atomic
     * operation on "tail", so there is a simple atomic point of
     * demarcation from unqueued to queued. Similarly, dequeuing
     * involves only updating the "head". However, it takes a bit
     * more work for nodes to determine who their successors are,
     * in part to deal with possible cancellation due to timeouts
     * and interrupts.
     *
     * <p>The "prev" links (not used in original CLH locks), are mainly
     * needed to handle cancellation. If a node is cancelled, its
     * successor is (normally) relinked to a non-cancelled
     * predecessor. For explanation of similar mechanics in the case
     * of spin locks, see the papers by Scott and Scherer at
     * http://www.cs.rochester.edu/u/scott/synchronization/
     *
     * <p>We also use "next" links to implement blocking mechanics.
     * The thread id for each node is kept in its own node, so a
     * predecessor signals the next node to wake up by traversing
     * next link to determine which thread it is.  Determination of
     * successor must avoid races with newly queued nodes to set
     * the "next" fields of their predecessors.  This is solved
     * when necessary by checking backwards from the atomically
     * updated "tail" when a node's successor appears to be null.
     * (Or, said differently, the next-links are an optimization
     * so that we don't usually need a backward scan.)
     *
     * <p>Cancellation introduces some conservatism to the basic
     * algorithms.  Since we must poll for cancellation of other
     * nodes, we can miss noticing whether a cancelled node is
     * ahead or behind us. This is dealt with by always unparking
     * successors upon cancellation, allowing them to stabilize on
     * a new predecessor, unless we can identify an uncancelled
     * predecessor who will carry this responsibility.
     *
     * <p>CLH queues need a dummy header node to get started. But
     * we don't create them on construction, because it would be wasted
     * effort if there is never contention. Instead, the node
     * is constructed and head and tail pointers are set upon first
     * contention.
     *
     * <p>Threads waiting on Conditions use the same nodes, but
     * use an additional link. Conditions only need to link nodes
     * in simple (non-concurrent) linked queues because they are
     * only accessed when exclusively held.  Upon await, a node is
     * inserted into a condition queue.  Upon signal, the node is
     * transferred to the main queue.  A special value of status
     * field is used to mark which queue a node is on.
     *
     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
     * Scherer and Michael Scott, along with members of JSR-166
     * expert group, for helpful ideas, discussions, and critiques
     * on the design of this class.
     */
    static final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        //可以理解为枚举的共享模式
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        //枚举的独占模式
        static final Node EXCLUSIVE = null;

        //以下为状态可选的值
        /** waitStatus value to indicate thread has cancelled */
        //表示 当前节点处于取消状态
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        //表示 当前节点需要唤醒他的后继节点  signal表示的其实是后继节点的状态
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition */
        //
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        //
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values:
         *   SIGNAL:     The successor of this node is (or will soon be)
         *               blocked (via park), so the current node must
         *               unpark its successor when it releases or
         *               cancels. To avoid races, acquire methods must
         *               first indicate they need a signal,
         *               then retry the atomic acquire, and then,
         *               on failure, block.
         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
         *               Nodes never leave this state. In particular,
         *               a thread with cancelled node never again blocks.
         *   CONDITION:  This node is currently on a condition queue.
         *               It will not be used as a sync queue node
         *               until transferred, at which time the status
         *               will be set to 0. (Use of this value here has
         *               nothing to do with the other uses of the
         *               field, but simplifies mechanics.)
         *   PROPAGATE:  A releaseShared should be propagated to other
         *               nodes. This is set (for head node only) in
         *               doReleaseShared to ensure propagation
         *               continues, even if other operations have
         *               since intervened.
         *   0:          None of the above
         *
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn't need to
         * signal. So, most code doesn't need to check for particular
         * values, just for sign.
         *
         * The field is initialized to 0 for normal sync nodes, and
         * CONDITION for condition nodes.  It is modified using CAS
         * (or when possible, unconditional volatile writes).
         * 当前node的状态  可选值就是上面的4个以及0  即：
         * 0、CANCELLED(-1)、SIGNAL(1)、CONDITION(2)、PROPAGATE(3)
         *
         * waitStatus == 0 默认状态
         * waitStatus > 0 取消状态
         * waitStatus == -1 表示当前node如果是head节点时,释放锁之后需要唤醒他的后继节点
         */
        volatile int waitStatus;

        //由于node要构建出一个fifo的队列，所以需要前驱节点以及后继节点
        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         * 前驱节点
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         * 后继节点
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         * 当前node封装的线程本尊
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred to the queue to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         */
        //ReentrantLock 中没有使用到
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null.
         * Use when predecessor cannot be null.  The null check could
         * be elided, but is present to help the VM.
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        Node() {    // Used to establish initial head or SHARED marker
        }

        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     *
     * FIFO队列的头节点
     * 任何时刻，头节点对应的线程都是当前持有锁的线程
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     *
     * 阻塞队列的尾节点
     * 阻塞队列不包含头节点，因为头节点处于活跃状态
     * 一般认为 head.next ---------------> tail 为阻塞队列
     */
    private transient volatile Node tail;

    /**
     * The synchronization state.
     *
     * 表示资源
     * 独占模式下： 0表示未加锁状态   >0 表示已经加锁状态
     */
    private volatile int state;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    private Node enq(final Node node) {
        //自旋入队，  只有当前node入队成功，才会跳出
        for (;;) {
            //获取尾节点
            Node t = tail;
            //1、表示当前队列是空队列
            //说明当前锁 被占用，且当前线程可能是第一个获取锁失败的线程(因为可能会有一批都来到这里)
            if (t == null) { // Must initialize
                //作为当前持有锁线程的第一个  后继线程  需要做以下几个事情:
                //1、初始化头尾节点，因为第一个线程持有锁成功之后，没有初始化阻塞队列  没有添加任何node
                //2、为自己追加node，也就是自己入队

                //CAS 成功  表示当前线程  变成了 head.next节点
                //线程需要为持有锁的线程创建head节点
                if (compareAndSetHead(new Node()))
                    tail = head;
                //这里并没有返回，所以初始化成功与失败都不会有其他操作
                //当前线程还  必须自己竞争入队才行
            } else {
                //普通入队方式  和快速入队没有任何区别  只不过这里加了自选，会保证入队成功并且返回node
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        //封装当前线程到node节点中  并且赋值节点模式
        //AQS 默认使用独占模式的锁
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        //尝试快速入队，如果入队成功直接返回，入队失败再走完成的入队流程
        //获取队尾节点，并保存到pred变量中
        //这个时候 pred节点就相当于我们的前置节点
        Node pred = tail;
        //条件成立，表示队列中已经有节点了，有节点才可以快速入队
        if (pred != null) {
            //快速入队过程:
            //1、先把自己的前一个节点设置为当前的尾节点
            //2、尝试把尾节点指向自己，指向成功，表示入队成功
            //3、如果入队成功，那么把前一个节点的 next引用指向自己
            node.prev = pred;
            //尝试设置自己为 尾节点
            if (compareAndSetTail(pred, node)) {
                //前置节点的 next 指向自己
                pred.next = node;
                return node;
            }
        }
        //快速入队失败   需要走完整的入队
        //指向到这里的集中情况
        //1、当前队列是空队列
        //2、CAS失败   也就是快速入队失败/竞争入队失败
        enq(node);
        return node;
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     * 尝试唤醒节点的后继节点，如果后继节点存在
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        //获取当前节点的状态
        //0：  默认状态，也就是初始化状态
        //-1：  表示当前node如果是head节点时,释放锁之后需要唤醒他的后继节点
        //>0： 取消状态
        int ws = node.waitStatus;
        //表示当前节点 可以唤醒后继节点  所以要设置当前节点 状态为0
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        //获取当前节点的  next节点
        Node s = node.next;
        //后继节点大于0的时候  也就是后继节点要么不存在，要么后继节点也是取消状态
        if (s == null || s.waitStatus > 0) {
            //设置后继节点为null
            s = null;
            //从 尾节点遍历 双向链表 直到遍历到当前节点
            //尝试寻找到可以被唤醒的节点
            for (Node t = tail; t != null && t != node; t = t.prev)
                //找到了那个可以被唤醒的节点
                if (t.waitStatus <= 0)
                    //把找到的节点认为是需要被唤醒的那个节点
                    s = t;
        }

        //如果后继节点存在
        if (s != null)
            //尝试唤醒后继节点
            LockSupport.unpark(s.thread);
    }

    /**
     * 共享模式下  唤醒阻塞线程
     * 以CountDownLatch为例
     *
     * 什么时候会调用这个方法呢？
     * 1、latch.countDown() 刚好把state设置为0  这个线程会来调用这个方法，唤醒head.next节点
     * 2、被唤醒的线程  在await方法中醒来之后   会检查是否可以获取共享锁，如果可以获取那么就会 执行唤醒后继的逻辑  也就是调用本方法
     *
     * Release action for shared mode -- signals successor and ensures
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            //获取头节点
            Node h = head;
            //条件成立： 头节点不为空  并且  不是只有一个节点的情况下
            //在共享模式内  这个条件成立  代表有需要唤醒的节点
            //条件1不成立的情况：
            //latch创建出来之后，没有任何一个线程进入 await方法 之前
            //有线程  调用 latch.countDown 触发唤醒阻塞节点的逻辑
            //条件2不成立的情况：
            //1、正常唤醒逻辑下  依次获取到共享锁然后  唤醒下一个线程；  (当线程就是tail去唤醒后继的时候)
            //2、调用 await方法和 countDown方法并发了， 导致await方法还没有完全入队
            // countDown 方法就已经处理完唤醒逻辑了  而，await方法入队完成之后，会判断是否可以获取锁
            // 获取到锁之后， 后面执行的await方法  就不会被阻塞了  会执行完自己的一个 设置头节点并唤醒后继的逻辑
            if (h != null && h != tail) {
                //执行到这里 说明当前阻塞队列  一定有后继节点需要唤醒
                //当前 头节点的 状态值
                int ws = h.waitStatus;
                //状态为 signal状态  可以唤醒后继  并且后继还没有被唤醒过
                if (ws == Node.SIGNAL) {
                    //通过CAS 把 头节点状态进行变更
                    //因为 最后一个判断可能让线程没有直接跳出 而唤醒的线程也会进入到这里来
                    //所以这里存在并发 所以使用CAS的方法来设置状态值
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    //头节点状态变更完成
                    //唤醒后继节点
                    unparkSuccessor(h);
                }
                //CountDownLatch  情况下 ws有且只有最后一个节点  也就是tail才会为0的状态
                //这个时候  设置  node状态为 -3  也就是传播状态
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }

            //执行到这里的两个情况：
            //1、存在后继节点，但是后继节点还没有把 自己设置为头节点，这个时候直接 推出唤醒逻辑
            //2、h == tail  也就是头节点和尾巴节点指向的是同一个对象  不存在后继，所以 h恒等于head 直接退出
            //在这里不需要担心 唤醒逻辑会被断掉，因为你换的后继 会继续执行唤醒逻辑
            //条件不成立的情况：
            //被唤醒的节点  在执行到下面之前  把自己设置为了新的head 这个时候 本次线程就无法跳出了  而唤醒的线程也会进入到这个方法里面来
            //经过自旋之后  就会有两个  线程来 唤醒 head.next节点了
            if (h == head)                   // loop if head changed
                break;
        }
    }

    /**
     * 设置当前节点为头节点并向后传播(依次唤醒)
     *
     * Sets head of queue, and checks if successor may be waiting
     * in shared mode, if so propagating if either propagate > 0 or
     * PROPAGATE status was set.
     *
     * @param node the node
     * @param propagate the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        //获取头节点
        Node h = head; // Record old head for check below
        //设置当前节点为头节点
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        //CountDownLatch情况下  调用这个方法来到这里的时候   propagate一定是1  所以条件1一定成立
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            //获取 当前节点的next节点
            Node s = node.next;
            //条件1成立：  当前节点的next是null  (当 s节点为  tail节点的时候成立)
            //条件2成立：  当前节点的next节点是共享模式的 (CountDownLatch的时候  所有节点全部是 共享模式的)
            if (s == null || s.isShared())
                //基本上所有情况都会  执行到这里来  CountDownLatch情况下
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     * 取消指定node 参与锁竞争的方法
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            //当前节点不存在的时候，直接出队
            return;

        //把参与竞争的线程制空
        //因为已经取消排队了，所以要清空node内部的当前线程引用
        node.thread = null;

        // Skip cancelled predecessors
        //获取当前  取消排队node的  前置节点
        Node pred = node.prev;

        //waitStatus的  几种情况：
        //0：  默认状态，也就是初始化状态
        //-1：  表示当前node如果是head节点时,释放锁之后需要唤醒他的后继节点
        //>0： 取消状态
        //前置节点是  取消状态，这个时候就去更前的节点，找一个非取消状态的节点
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        //拿到前置节点的  next 节点
        //有一下两种情况：
        //1、next节点是  当前节点
        //2、next节点  也是  canceled状态的节点
        //因为  我们只会越过 CANCELED状态的节点  去找非CANCELED节点
        //所以这个节点的后继节点  有且只有  CANCELED状态的节点  以及当前节点
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.

        //设置当前节点的状态为 取消状态  当其他正常节点被唤醒之后，会帮助取消状态的节点出队
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.

        /**
         * 当前取消排队的node所在 队列中的位置不同  执行的出队逻辑策略是不同的  一共有三种情况
         * 1、当前node是队尾  tail -> node
         * 2、当前node  不是 head.next 节点  也不是 tail节点
         * 3、当前node 是  head.next 节点
         */

        //条件成立：  表示当前节点的尾节点   并且  设置尾节点为前置节点成功
        //表示自己是尾节点   要把前置节点设置成了尾节点的情况
        if (node == tail && compareAndSetTail(node, pred)) {
            //把头节点的  next指针变成null  完成当前node的出队
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            /**
             * 来到这里的两种情况
             * 1、当前node  不是 head.next 节点  也不是 tail节点
             * 2、当前node 是  head.next 节点
             */
            //保存前置节点的状态
            int ws;
            //条件1： pred != head   条件成立 表示当前node不是 head.next 也不是tail
            //条件2.1：(ws = pred.waitStatus) == Node.SIGNAL
            //条件成立 表示前一个节点状态 是 Single状态的
            //条件不成立：前一个节点 状态不是 Single的   可能是0
            //条件2.2：(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL)
            //条件成立 表示前一个节点不是 SIGNAL状态  但是设置其节点状态为SIGNAL状态成功
            //这里 前置节点的状态  绝大多数情况下都是 =0的情况  所以可以直接设置为 SIGNAL状态
            //如果前置节点>0  这里设置为 -1 是为了可以唤醒后继节点
            //但是极端情况下  前置节点也取消了 这个时候还是会 尝试把前置节点的状态设置为 SIGNAL状态
            //条件3：pred.thread != null
            //条件成立表示  前置节点没有被取消
            //整个if条件就是保证 前置节点的状态是  SIGNAL的 可以唤醒后继节点 也就是当前被取消的节点
            //换句话来说就是要  保证有一个正常的前置节点，在前置节点处理的时候可以把自己处理出队
            //这样一套循环下来，就可以保证 每个要出队的节点 都选择过一个可以帮助他出队的节点
            //这样就可以把所有要出队的节点相当于都以一个头插法的方式  插到正常节点后面
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                //情况2： 当前node  不是 head.next 节点  也不是 tail节点
                //出队： pred.next -> node.next 节点后, 当node.next 节点 被唤醒后
                //调用 shouldParkAfterFailedAcquire 会让 node.next节点越过取消状态的节点
                //获取当前节点的后继节点
                Node next = node.next;
                //条件成立  表示当前节点的后继节点是正常状态的节点
                if (next != null && next.waitStatus <= 0)
                    //把 当前节点的后继节点 设置到  当前节点 前置节点的 next指针上
                    //这个操作会在 node.next节点调用 shouldParkAfterFailedAcquire  越过取消状态的节点
                    //那个时候会让这些取消的节点全部出队
                    //而如果没有走到这里，表示 node.next 要么状态是取消状态
                    //要么是为null的，这个一般不会出现，极端情况可能出现
                    //出现了以上两种情况就不需要设置next节点了
                    compareAndSetNext(pred, predNext, next);
            } else {
                //情况3：当前node 是  head.next 节点
                //类似情况2， 后继节点唤醒之后  会调用 shouldParkAfterFailedAcquire 越过被取消的节点
                //队列的第三个节点会 直接与 head 建立双重指向的关系  head.next -> 第三个node
                //上面的逻辑会在  节点唤醒之后执行
                //也就是说  这里只是尝试唤醒后面可能需要唤醒的节点
                unparkSuccessor(node);
            }
            //清空 当前节点的 next指针 提高GC
            node.next = node; // help GC
        }
    }

    /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev.
     *
     * 总结：
     * 1、当前节点的前置节点 状态为 CANCELED 第一次来到这个方法时，会越过取消节点，第二次会返回true并 park当前线程
     * 2、当前节点的前置节点 状态为 0  当前线程会设置前置节点的状态为-1  提醒前置节点要唤醒后继节点  第二次来到这里时和上面一样
     * 3、当前节点的前置节点 状态为-1  返回true park当前线程
     *
     *
     * @param pred node's predecessor holding status  当前线程node的前置节点
     * @param node the node   当前线程对应的node
     * @return {@code true} if thread should block  true表示当前线程需要挂起，反之则不需要
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        //获取前置节点的状态
        //waitStatus的集中情况
        //0  默认状态 new node()
        //-1  signal状态  表示当前节点释放锁之后,会唤醒它的第一个后继节点
        //>0   表示当前节点是 CANCELED状态
        int ws = pred.waitStatus;
        //条件成立，表示前置节点 是一个可以唤醒当前节点的节点，可以挂起返回true
        //一般情况下，第一次来到这里的时候   ws不会是 -1
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
            //条件成立：表示前置节点是 CANCELED状态  这个状态无法唤醒后继节点
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            //不断的寻找可以唤醒自己的节点，并把自己设置到对应的节点后面
            //这个过程中会把  状态是  CANCELED的节点全部出队  因为这个状态的节点无法被遍历到了
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            //前置节点的 ws是0的情况  直接把前置节点的 ws值设置为 SIGNAL状态
            //表示前置节点释放锁之后，需要唤醒第一个后继节点
            //一般发生在  新节点入队之后 前一个节点还没有设置状态的时候
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     * 中断当前线程
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * Convenience method to park and then check if interrupted
     * 挂起当前线程，唤醒后返回当前线程 是否为中断唤醒
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        //挂起当前线程
        LockSupport.park(this);
        //唤醒之后，返回当前线程的中断标记
        return Thread.interrupted();
    }

    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     * 1、判断当前节点有没有被挂起  也就是 park掉
     * 如果没有  则挂起操作
     * 2、唤醒之后的相关逻辑
     *
     * @param node the node  当前线程封装出的node   并且当前线程已经入队成功
     * @param arg the acquire argument   当前线程抢占资源成功，设置的state值会用到   所以这里传入
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        //true  表示当前线程抢占锁成功    一般情况下(lock) 当前线程早晚会拿到锁
        //false 表示失败，需要执行出队的逻辑
        boolean failed = true;
        try {
            //当前线程是否被中断
            boolean interrupted = false;
            //自旋
            for (;;) {
                //获取当前节点的前置节点
                final Node p = node.predecessor();
                //判断当前节点是不是头节点后面的第一个节点  如果是 当前线程有权限和头节点竞争锁资源
                //条件不成立:
                //1、前置节点不是头节点，需要挂起 park掉
                //2、头节点还没有释放锁，这个时候获取锁失败
                if (p == head && tryAcquire(arg)) {
                    //条件成立  表示当前节点竞争锁资源成功  head对应的线程已经释放锁了
                    //设置当前节点为头节点
                    setHead(node);
                    //把之前头节点的next 指针设置为null  变相给头节点出队 并 帮助GC
                    p.next = null; // help GC
                    //设置为 false 表示当前线程出队没有异常 (获取锁的过程中)
                    failed = false;
                    //返回 当前线程唤醒是被中断唤醒的还是自然唤醒的
                    return interrupted;
                }

                //shouldParkAfterFailedAcquire  这个方法是做什么的呢?
                //                  当前线程获取锁资源失败后，是否需要挂起
                //返回true：当前线程需要挂起
                //返回false：不需要挂起
                //parkAndCheckInterrupt()  挂起当前线程，当前线程被唤醒后返回当前线程的中断标记
                //中断之后，并返回当前线程的中断标记
                //唤醒：1、正常唤醒，其他线程 unpark 2、其他线程给当前挂起线程给了一个中断信号
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    //把中断信号设置为 true  表示当前线程被其他线程中断过，而非正常唤醒
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        //把当前线程封装成node  并入队
        final Node node = addWaiter(Node.EXCLUSIVE);
        //失败标记， 表示没有抢占锁成功
        boolean failed = true;
        try {
            //自旋
            for (;;) {
                //获取当前前置节点
                final Node p = node.predecessor();
                //如果是头节点则 尝试竞争锁
                if (p == head && tryAcquire(arg)) {
                    //竞争成功 设置当前线程为头节点 并帮助前置节点出队
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }

                //条件一：判断当前线程是否应该被挂起 会找到合适的前置节点才会返回true
                //条件成立表示 当前线程应该被挂起
                //条件二：挂起当前线程  当前线程被唤醒之后  返回中断标记
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    //如果当前线程是被中断唤醒的，那么抛出中断异常 响应中断
                    throw new InterruptedException();
            }
        } finally {
            //没有抢占锁成功，也不会来继续抢锁了，这个时候要执行出队的逻辑了
            //目前只有  响应中断的加锁才会有出队的逻辑
            if (failed)
                //取消竞争锁资源，然后当前节点出队
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * 响应中断的 获取共享锁的方式
     *
     * Acquires in shared uninterruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     *
     *
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        //把当前线程封装成 node  加入到 AQS阻塞队列中  模式为共享模式
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            //如果是共享模式，这里会发生什么呢？  以CountDownLatch举例
            //第一个进来的线程  是head节点  它无法获取到共享锁(因为资源不是0)  所以会被阻塞
            //第二个进来的线程  是head.next节点  它也无法获取锁  也会被阻塞
            //后面进来的所有线程  也就都会被阻塞在这里
            for (;;) {
                //获取当前节点的  前驱节点
                final Node p = node.predecessor();
                //判断  当前节点是不是头节点的  next节点
                if (p == head) {
                    //尝试获取共享锁   CountDownLatch中  如果state!=0(其实都是大于0的情况)  返回-1
                    int r = tryAcquireShared(arg);
                    //条件成立  表示共享锁获取成功  对应CountDownLatch中 state已经变成0了
                    //也就是  资源已经全部释放了
                    if (r >= 0) {
                        //进入这里  表示线程被唤醒后   state值已经变成0了
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                //shouldParkAfterFailedAcquire  给当前线程寻找一个可以唤醒自己的线程  从而保证自己阻塞之后可以被唤醒
                //parkAndCheckInterrupt  阻塞当前线程并发  中断标记返回
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    //两个条件成立  表示线程是被  中断唤醒的  所以抛出  中断异常
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // Main exported methods

    /**
     * Attempts to acquire in exclusive mode. This method should query
     * if the state of the object permits it to be acquired in the
     * exclusive mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread. This can be used
     * to implement method {@link Lock#tryLock()}.
     *
     * <p>The default
     * implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return {@code true} if successful. Upon success, this object has
     *         been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in exclusive
     * mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this object is now in a fully released
     *         state, so that any waiting threads may attempt to acquire;
     *         and {@code false} otherwise.
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *        passed to an acquire method, or is the value saved on entry
     *        to a condition wait.  The value is otherwise uninterpreted
     *        and can represent anything you like.
     * @return a negative value on failure; zero if acquisition in shared
     *         mode succeeded but no subsequent shared-mode acquire can
     *         succeed; and a positive value if acquisition in shared
     *         mode succeeded and subsequent shared-mode acquires might
     *         also succeed, in which case a subsequent waiting thread
     *         must check availability. (Support for three different
     *         return values enables this method to be used in contexts
     *         where acquires only sometimes act exclusively.)  Upon
     *         success, this object has been acquired.
     * @throws IllegalMonitorStateException if acquiring would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Attempts to set the state to reflect a release in shared mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this release of shared mode may permit a
     *         waiting acquire (shared or exclusive) to succeed; and
     *         {@code false} otherwise
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns {@code true} if synchronization is held exclusively with
     * respect to the current (calling) thread.  This method is invoked
     * upon each call to a non-waiting {@link ConditionObject} method.
     * (Waiting methods instead invoke {@link #release}.)
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}. This method is invoked
     * internally only within {@link ConditionObject} methods, so need
     * not be defined if conditions are not used.
     *
     * @return {@code true} if synchronization is held exclusively;
     *         {@code false} otherwise
     * @throws UnsupportedOperationException if conditions are not supported
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     */
    public final void acquire(int arg) {
        //条件1：tryAcquire 尝试获取锁  获取成功返回 true  获取失败返回false
        //条件2：
        // 2.1 addWaiter  将当前线程封装成node入队
        // 2.2 acquireQueued 挂起当前线程   唤醒后相关的逻辑都在这里;  返回true 表示挂起过程中线程被中断唤醒过  返回false  表示未被中断过
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            //再次设置中断标记为true
            //进入这里表示  当前线程被中断唤醒过  在这里再次中断执行业务逻辑
            //如果加锁的代码并没有响应中断，那么什么都不会发生
            selfInterrupt();
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        //如果线程中断标记变成了  被中断这个时候抛出中断异常
        if (Thread.interrupted())
            throw new InterruptedException();
        //尝试加锁
        if (!tryAcquire(arg))
            //竞争锁失败
            doAcquireInterruptibly(arg);
    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted,
     * and failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquire}, returning on success.  Otherwise, the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted
     * or the timeout elapses.  This method can be used to implement
     * method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     * 释放锁的方法
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        //条件成立，释放锁成功
        if (tryRelease(arg)) {
            //获取头节点
            Node h = head;
            //条件成立： 头节点不为空
            //并且头节点的状态是非正常状态的  释放完之后，只有0是 什么都不需要做的
            //0：  默认状态，也就是初始化状态
            //-1：  表示当前node如果是head节点时,释放锁之后需要唤醒他的后继节点
            //>0： 取消状态
            if (h != null && h.waitStatus != 0)
                //在这个方法里面，如果 状态是 -1 尝试唤醒后继
                //如果状态 >0 会尝试从后面找到一个没有被取消的节点，从而唤醒他
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * Acquires in shared mode, ignoring interrupts.  Implemented by
     * first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    /**
     * Acquires in shared mode, aborting if interrupted.  Implemented
     * by first checking interrupt status, then invoking at least once
     * {@link #tryAcquireShared}, returning on success.  Otherwise the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted.
     * @param arg the acquire argument.
     * This value is conveyed to {@link #tryAcquireShared} but is
     * otherwise uninterpreted and can represent anything
     * you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        //条件成立 表示 线程已经被中断  抛出异常处理
        if (Thread.interrupted())
            throw new InterruptedException();
        //CountDownLatch中  条件成立表示 state值 >0  需要把线程入队等待
        //也就是 可以阻塞   如果state值=0  表示count数已经满了  不需要入队阻塞等待了
        if (tryAcquireShared(arg) < 0)
            //把线程入队阻塞等待
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and
     * failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquireShared}, returning on success.  Otherwise, the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted or the timeout elapses.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException if the current thread is interrupted
     */
    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }

    /**
     * 释放共享锁
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
        //以CountDownLatch为例  条件成立  表示当前调用 latch.countDown() 的线程  刚好  state-1 == 0
        //这个线程需要触发唤醒  await状态线程的  逻辑
        if (tryReleaseShared(arg)) {
            //进入这里  表示共享锁已经释放完毕  也就是  state=0了  在CountDownLatch中
            //在这里  只有一个线程会进入  进入之后 会执行唤醒其他阻塞状态线程的逻辑
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

    /**
     * Queries whether any threads are waiting to acquire. Note that
     * because cancellations due to interrupts and timeouts may occur
     * at any time, a {@code true} return does not guarantee that any
     * other thread will ever acquire.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    public final boolean hasQueuedThreads() {
        return head != tail;
    }

    /**
     * Queries whether any threads have ever contended to acquire this
     * synchronizer; that is if an acquire method has ever blocked.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there has ever been contention
     */
    public final boolean hasContended() {
        return head != null;
    }

    /**
     * Returns the first (longest-waiting) thread in the queue, or
     * {@code null} if no threads are currently queued.
     *
     * <p>In this implementation, this operation normally returns in
     * constant time, but may iterate upon contention if other threads are
     * concurrently modifying the queue.
     *
     * @return the first (longest-waiting) thread in the queue, or
     *         {@code null} if no threads are currently queued
     */
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if (((h = head) != null && (s = h.next) != null &&
             s.prev == head && (st = s.thread) != null) ||
            ((h = head) != null && (s = h.next) != null &&
             s.prev == head && (st = s.thread) != null))
            return st;

        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */

        Node t = tail;
        Thread firstThread = null;
        while (t != null && t != head) {
            Thread tt = t.thread;
            if (tt != null)
                firstThread = tt;
            t = t.prev;
        }
        return firstThread;
    }

    /**
     * Returns true if the given thread is currently queued.
     *
     * <p>This implementation traverses the queue to determine
     * presence of the given thread.
     *
     * @param thread the thread
     * @return {@code true} if the given thread is on the queue
     * @throws NullPointerException if the thread is null
     */
    public final boolean isQueued(Thread thread) {
        if (thread == null)
            throw new NullPointerException();
        for (Node p = tail; p != null; p = p.prev)
            if (p.thread == thread)
                return true;
        return false;
    }

    /**
     * Returns {@code true} if the apparent first queued thread, if one
     * exists, is waiting in exclusive mode.  If this method returns
     * {@code true}, and the current thread is attempting to acquire in
     * shared mode (that is, this method is invoked from {@link
     * #tryAcquireShared}) then it is guaranteed that the current thread
     * is not the first queued thread.  Used only as a heuristic in
     * ReentrantReadWriteLock.
     * 判断head.next节点是否是独占模式等待者，如果是  返回true，如果不是返回false
     */
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        //条件1成立：   表示当前阻塞队列 head节点不是null
        //条件2成立：   表示当前阻塞队列 head.next节点不是null
        //条件3成立：   head.next节点不是共享模式的
        //条件4成立：   表示 head.next节点 是一个可以被唤醒的节点
        return (h = head) != null &&
            (s = h.next)  != null &&
            !s.isShared()         &&
            s.thread != null;
    }

    /**
     * Queries whether any threads have been waiting to acquire longer
     * than the current thread.
     *
     * <p>An invocation of this method is equivalent to (but may be
     * more efficient than):
     *  <pre> {@code
     * getFirstQueuedThread() != Thread.currentThread() &&
     * hasQueuedThreads()}</pre>
     *
     * <p>Note that because cancellations due to interrupts and
     * timeouts may occur at any time, a {@code true} return does not
     * guarantee that some other thread will acquire before the current
     * thread.  Likewise, it is possible for another thread to win a
     * race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     *
     * <p>This method is designed to be used by a fair synchronizer to
     * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
     * Such a synchronizer's {@link #tryAcquire} method should return
     * {@code false}, and its {@link #tryAcquireShared} method should
     * return a negative value, if this method returns {@code true}
     * (unless this is a reentrant acquire).  For example, the {@code
     * tryAcquire} method for a fair, reentrant, exclusive mode
     * synchronizer might look like this:
     *
     *  <pre> {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}</pre>
     *
     * @return {@code true} if there is a queued thread preceding the
     *         current thread, and {@code false} if the current thread
     *         is at the head of the queue or the queue is empty
     * @since 1.7
     */
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }


    // Instrumentation and monitoring methods

    /**
     * Returns an estimate of the number of threads waiting to
     * acquire.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring system state, not for synchronization
     * control.
     *
     * @return the estimated number of threads waiting to acquire
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its state.
     * The state, in brackets, includes the String {@code "State ="}
     * followed by the current value of {@link #getState}, and either
     * {@code "nonempty"} or {@code "empty"} depending on whether the
     * queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    public String toString() {
        int s = getState();
        String q  = hasQueuedThreads() ? "non" : "";
        return super.toString() +
            "[State = " + s + ", " + q + "empty queue]";
    }


    // Internal support methods for Conditions

    /**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     * @param node the node
     * @return true if is reacquiring
     * 判断当前节点是在  阻塞队列中  还是条件队列中
     * 如果在条件队列中  返回  false
     * 如果在阻塞队列中  返回  true
     */
    final boolean isOnSyncQueue(Node node) {
        //条件1：node.waitStatus == Node.CONDITION  条件成立  表示当前节点一定在 条件队列中
        //因为 signal方法迁移到 阻塞队列之前 就会修改 node的状态  所以状态为 CONDITION 一定在条件队列
        //条件2： node.prev == null   前置条件： node的状态不是 CONDITION了
        //前置条件 的集中情况：
        //1、node.waitStatus == 0  表示当前节点已经被signal了
        //2、node.waitStatus == 1  表示当前线程是 未持有锁 调用await方法 最终将 node状态改成了取消状态
        // node.waitStatus == 0  为什么还要判断条件2？
        //因为 signal方法是 先修改状态  后迁移的  所以这里进行了判断
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;

        //执行到这里的集中情况：
        // node.waitStatus != CONDITION && node.prev != null
        //可以排除掉  node.waitStatus == 1 的情况
        //为什么可以排除？  因为 signal 方法是不会把 取消状态的节点迁移走的  是直接出队代替
        //设置 prev引用的逻辑在阻塞队列中 是  入队 enq() 方法做的
        //入队逻辑：
        //1、 node.prev = tail
        //2、 CASSet 当前为 队尾
        //3、 tail.next = node
        //只有全部执行完才算入队  所以 prev != null 不代表已经入队了
        //所以  就算 prev != null  也不能说明当前node 已经成功入队了

        //next 指针不为空  条件队列不会使用 next指针
        //也只有阻塞队列 会 使用next指针
        //而这个时候  prev != null；   next != null 则表示节点已经正常入队了
        if (node.next != null) // If has successor, it must be on queue
            return true;

        /*
         * 执行到这里：
         * 表示当前node  状态不是CONDITION && (node.prev != null)  && (node.next == null)
         * 也就是当前node状态 是 0 也就是新建状态
         * 这个时候  可能是 signal进行中  正在迁移节点  还未完成
         * 也有可能是  CAS竞争失败了(可能性比较低)  当前节点还在和 其他线程竞争入队
         * 所以需要从 队尾进行查找以确认当前节点是否在  队列中
         */

        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     * 从阻塞队列队尾查找当前节点  如果找到返回 true  如果没有找到 返回false
     */
    private boolean findNodeFromTail(Node node) {
        //获取尾节点
        Node t = tail;
        //自旋遍历
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     * 把节点从  条件队列  迁移到  阻塞队列中的方法
     *
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        //CAS修改 当前节点的状态  为0
        //CAS 成功：  表示节点在 条件队列中的状态正常
        //CAS 失败：  1、表示节点在  条件队列中已经被取消  无法迁移
        //           2、表示 node对应的线程挂起期间  被其他线程中断唤醒(这个时候 会从条件队列迁移到阻塞队列中  也会修改状态为0)
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */

        //将当前节点入队  并返回入队后的前一个节点
        Node p = enq(node);
        //获取  前一个节点 的状态值
        int ws = p.waitStatus;
        // ws > 0 表示节点状态是取消状态
        // compareAndSetWaitStatus(p, ws, Node.SIGNAL)  CAS修改 当前状态为SIGNAL状态  这个状态表示可以唤醒后继节点
        // CAS  返回 false 的情况：
        // 当前驱节点 对应的线程使用 响应中断的加锁方式 加锁时，  外部线程给了 前驱节点中断信号之后  前驱节点 会将状态改为 CANCEL状态
        // 然后执行出队逻辑
        // 只要前驱节点 无法唤醒当前线程(状态不是 0 或-1) 那么就需要唤醒当前线程
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            //唤醒当前线程
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait.
     * Returns true if thread was cancelled before being signalled.
     *
     * @param node the node
     * @return true if cancelled before the node was signalled
     */
    final boolean transferAfterCancelledWait(Node node) {
        //条件队列中  中断状态 迁移  阻塞队列操作
        //条件成立：  表示当前节点一定在 条件队列中  并且迁移到阻塞队列中  所以需要设置状态为0
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            //把当前节点添加到条件队列中
            enq(node);
            //返回true  表示是在 条件队列中被中断的
            return true;
        }

        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        //执行到这里的几种情况？
        //1、当前node 已经被外部线程调用 signal 方法迁移到 阻塞队列中了
        //2、当前node 正在被外部线程调用 signal 方法迁移到阻塞队列  进行中

        //这个时候 如果还没有迁移完，那就需要等待  并且释放一点CPU，帮助迁移到阻塞队列中
        while (!isOnSyncQueue(node))
            Thread.yield();
        //返回 false 表示当前节点 被中断唤醒时  不在条件队列中
        return false;
    }

    /**
     * 释放当前节点(线程) 所有占用的锁资源
     *
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        //当前node 完全释放锁  是否成功
        //如果是 true 表示当前线程 是未持有锁的线程  然后去调用的 await()方法
        //这个属于  操作错误，也就是使用错误  写法错误
        boolean failed = true;
        try {
            //获取当前线程 所持有的 state 值 总数
            int savedState = getState();
            //绝大部分情况下  都会返回true
            //(另外的情况是返回 false  ReentrantLock中的实现)
            if (release(savedState)) {
                //释放锁成功
                failed = false;
                //返回 当前线程  占用的锁资源 state值
                //方便重新唤醒后  恢复现场
                //如果不恢复现场  一旦出现重入的情况就会导致 整个释放锁失败导致异常
                return savedState;
            } else {
                //完全释放锁失败 抛出异常
                throw new IllegalMonitorStateException();
            }
        } finally {
            //释放锁失败，节点状态变为取消状态  当他的后继节点会帮助他 出 条件队列
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

    // Instrumentation methods for conditions

    /**
     * Queries whether the given ConditionObject
     * uses this synchronizer as its lock.
     *
     * @param condition the condition
     * @return {@code true} if owned
     * @throws NullPointerException if the condition is null
     */
    public final boolean owns(ConditionObject condition) {
        return condition.isOwnedBy(this);
    }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this synchronizer. Note that because timeouts
     * and interrupts may occur at any time, a {@code true} return
     * does not guarantee that a future {@code signal} will awaken
     * any threads.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @param condition the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, the
     * estimate serves only as an upper bound on the actual number of
     * waiters.  This method is designed for use in monitoring of the
     * system state, not for synchronization control.
     *
     * @param condition the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this
     * synchronizer.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate. The elements of the
     * returned collection are in no particular order.
     *
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if exclusive synchronization
     *         is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this synchronizer
     * @throws NullPointerException if the condition is null
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }

    /**
     * Condition implementation for a {@link
     * AbstractQueuedSynchronizer} serving as the basis of a {@link
     * Lock} implementation.
     *
     * <p>Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * {@code AbstractQueuedSynchronizer}.
     *
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        /**
         * 指向条件队列的第一个节点
         */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        /**
         * 指向条件队列的最后一个节点
         */
        private transient Node lastWaiter;

        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() { }

        // Internal methods

        /**
         * Adds a new waiter to wait queue.
         * @return its new wait node
         *
         * 调用 await方法的线程肯定都是持有锁的线程
         * 也就是 这里一定不存在并发
         */
        private Node addConditionWaiter() {
            //获取最后一个  等待者节点
            Node t = lastWaiter;

            // If lastWaiter is cancelled, clean out.
            //条件成立：
            //条件队列的队尾节点不为空   并且   该节点状态不是  CONDITION
            //条件1成立：  表示当前条件队列 已经有节点了追加就可以了
            //条件2成立： 表示当前t这个节点  状态不是 CONDITION
            //在 条件队列中  如果不是 CONDITION状态 则说明当前节点状态已经变成了 CANCEL状态
            if (t != null && t.waitStatus != Node.CONDITION) {
                //进入这里面表示  当前条件队列不为空  并且存在取消状态的节点
                //把所有取消状态的  节点全部 清理出 条件队列
                unlinkCancelledWaiters();
                //更新 lastWaiter引用  因为 清理条件队列的时候  可能会更改 lastWaiter的引用
                t = lastWaiter;
            }

            //封装当前线程为node  设置  状态为  CONDITION
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            //如果条件队列的  最后一个节点是  null  那么当前节点就是  第一个进入条件队列的 节点
            if (t == null)
                firstWaiter = node;
            //如果不是  那么就把当前节点设置为   最后一个节点的   next节点
            else
                t.nextWaiter = node;
            //设置当前节点为  条件队列的最后一个节点
            lastWaiter = node;
            //返回当前线程封装的node
            return node;
        }

        /**
         * Removes and transfers nodes until hit non-cancelled one or
         * null. Split out from signal in part to encourage compilers
         * to inline the case of no waiters.
         * @param first (non-null) the first node on condition queue
         * 迁移条件队列中  第一个节点到阻塞队列中
         * 在这里也不会有  并发   因为只有只有锁线程才能调用 signal方法
         */
        private void doSignal(Node first) {
            do {
                //条件成立   当前条件队列中只有一个节点
                if ( (firstWaiter = first.nextWaiter) == null)
                    //设置最后一个  节点  也为 空
                    lastWaiter = null;
                //清空  当前节点的 nextWaiter  指针
                //first节点 出条件队列
                first.nextWaiter = null;
                //transferForSignal(first)
                //返回 true 表示迁移到阻塞队列成功
                //返回 false  表示迁移到阻塞队列失败

                //如果迁移到阻塞队列失败  更新first 为他的nextWaiter节点
                //循环遍历 迁移  直到有一个节点被迁移  或  条件队列为空
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

        /**
         * 因为持有锁，所以这里不会有并发
         * 唤醒所有节点
         * Removes and transfers all nodes.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignalAll(Node first) {
            //清空 第一个最后一个节点引用
            lastWaiter = firstWaiter = null;
            //循环遍历 迁移节点到阻塞队列中
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * Unlinks cancelled waiter nodes from condition queue.
         * Called only while holding lock. This is called when
         * cancellation occurred during condition wait, and upon
         * insertion of a new waiter when lastWaiter is seen to have
         * been cancelled. This method is needed to avoid garbage
         * retention in the absence of signals. So even though it may
         * require a full traversal, it comes into play only when
         * timeouts or cancellations occur in the absence of
         * signals. It traverses all nodes rather than stopping at a
         * particular target to unlink all pointers to garbage nodes
         * without requiring many re-traversals during cancellation
         * storms.
         * 清理当前条件队列中所有取消状态的节点
         * ps: 因为正常状态的节点都会被迁移到阻塞队列中，所以条件队列中节点的状态只有两个，
         * 一个是取消状态  另一个是 CONDITION状态
         */
        private void unlinkCancelledWaiters() {
            //获取当前条件队列  第一个节点
            Node t = firstWaiter;
            //trail  记录最后一个  正常状态的节点（当前链表正常状态的一个节点）
            Node trail = null;
            //遍历当前 条件队列
            while (t != null) {
                //获取当前节点的  next节点
                Node next = t.nextWaiter;
                //条件成立 表示当前节点状态  是取消状态  需要清理
                if (t.waitStatus != Node.CONDITION) {
                    //清空当前  t的引用
                    t.nextWaiter = null;
                    //表示前面的节点 都是取消状态的节点  没有正常状态的节点
                    if (trail == null)
                        //更新  条件队列第一个节点的引用
                        firstWaiter = next;
                    //最近一个正常状态的节点 不为空
                    else
                        trail.nextWaiter = next;
                    //表示已经遍历到最后一个节点
                    if (next == null)
                        //更新 条件队列最后一个节点的引用 为最后一个正常节点的引用
                        lastWaiter = trail;
                }
                else
                    //当前节点 状态正常  赋值回  trail
                    trail = t;
                //遍历下一个节点
                t = next;
            }
        }

        // public methods

        /**
         * Moves the longest-waiting thread, if one exists, from the
         * wait queue for this condition to the wait queue for the
         * owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        public final void signal() {
            //判断当前线程是否是  独占锁的持锁线程
            if (!isHeldExclusively())
                //当前线程不是  持有锁的线程  抛出异常
                throw new IllegalMonitorStateException();
            //获取条件队列的第一个节点
            Node first = firstWaiter;
            //如果条件队列的第一个节点不为空
            if (first != null)
                //将 第一个节点 迁移到阻塞队列的中的逻辑
                doSignal(first);
        }

        /**
         * 唤醒全部节点
         *
         * Moves all threads from the wait queue for this condition to
         * the wait queue for the owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        public final void signalAll() {
            //判断当前线程是否是持有锁的线程，不是异常处理
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            //获取第一个节点
            Node first = firstWaiter;
            //第一个节点不为空 唤醒其后面的全部节点
            if (first != null)
                doSignalAll(first);
        }

        /**
         * Implements uninterruptible condition wait.
         * <ol>
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * </ol>
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

        /*
         * For interruptible waits, we need to track whether to throw
         * InterruptedException, if interrupted while blocked on
         * condition, versus reinterrupt current thread, if
         * interrupted while blocked waiting to re-acquire.
         */

        /** Mode meaning to reinterrupt on exit from wait */
        private static final int REINTERRUPT =  1;
        /** Mode meaning to throw InterruptedException on exit from wait */
        private static final int THROW_IE    = -1;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted
         * before signalled, REINTERRUPT if after signalled, or
         * 0 if not interrupted.
         */
        private int checkInterruptWhileWaiting(Node node) {
            //Thread.interrupted()  返回当前线程的中断标记位  并且重置标记位 为false
            //transferAfterCancelledWait(node)  只有在线程被中断的情况下  才会被调用
            return Thread.interrupted() ?
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

        /**
         * Throws InterruptedException, reinterrupts current thread, or
         * does nothing, depending on mode.
         * 中断处理
         */
        private void reportInterruptAfterWait(int interruptMode)
            throws InterruptedException {
            //条件成立：说明在条件队列内，发生了中断，此时 await方法抛出中断异常
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            //条件成立  说明在条件队列外发生的中断  此时设置中断标记位 为true
            //把中断 交给  业务代码进行处理
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled or interrupted.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final void await() throws InterruptedException {
            //判断当前线程是否被中断过  如果中断过抛出中断异常
            if (Thread.interrupted())
                throw new InterruptedException();
            //把当前线程封装成 Condition中的 node节点 使其进入条件队列  然后返回node节点引用
            Node node = addConditionWaiter();

            //释放全部的锁  并记录占用的资源
            //如果没有释放锁  别人没有办法进来唤醒你了   记录占用的资源就是为了后面可以恢复现场
            //当前线程释放锁之后  会唤醒他的后继节点  他的后继节点会帮助当前线程  出 阻塞队列
            //从而实现  上面的代码先 进入条件队列  下面的代码立马离开  阻塞队列
            int savedState = fullyRelease(node);

            //0 表示 在 condition队列中 挂起的时候 未接受到中断信号
            //-1 表示 在condition队列 挂起期间  接收到中断信号了
            //1 表示  在condition队列 挂起期间  未接收到中断信息，但是 signal迁移到 阻塞队列之后 接受过中断信号
            int interruptMode = 0;

            //isOnSyncQueue(node)  判断当前节点是否在阻塞队列中
            //如果是 返回true  如果不是 返回false
            //返回 false表示 当前node还在条件队列中，需要继续挂起
            while (!isOnSyncQueue(node)) {
                //挂起当前线程
                LockSupport.park(this);
                //checkInterruptWhileWaiting  就算在condition条件队列  挂起期间   线程发生中断了
                //对应的node节点 还是会被迁移到 阻塞队列中

                //线程被唤醒的几种情况：
                //1、正常情况，外部线程获取到lock之后 调用signal方法 迁移条件队列头节点到  阻塞队列中；
                // 当这个节点的前驱节点释放锁之后  会唤醒
                //2、迁移 到 阻塞队列后  发现阻塞队列中前驱节点 是取消状态的  这个时候会唤醒
                //3、当线程挂起 期间  被其他外部线程使用中断唤醒
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }

            //执行到这里  就表示当前线程/node节点  已经被  迁移到 阻塞队列中了

            //acquireQueued  竞争队列的逻辑   条件成立：表示当前线程接受到 中断信号
            //interruptMode != THROW_IE  条件成立：说明 当前node在  条件队列内  未发生中断
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                //设置 中断模式为  重新中断
                interruptMode = REINTERRUPT;

            //条件成立的情况：
            //node在条件队列中的时候  如果被中断唤醒了  node会被直接添加到阻塞队列中  但是并没有在条件队列中出队
            //也就是 node.nextWaiter并没有置为 空
            if (node.nextWaiter != null) // clean up if cancelled
                //清理 条件队列内 取消状态的节点
                unlinkCancelledWaiters();

            //条件成立： 说明挂起期间，发生过中断
            //1、条件队列内的挂起
            //2、条件队列之外的挂起
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final long awaitNanos(long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return deadline - System.nanoTime();
        }

        /**
         * Implements absolute timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean awaitUntil(Date deadline)
                throws InterruptedException {
            long abstime = deadline.getTime();
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        //  support for instrumentation

        /**
         * Returns true if this condition was created by the given
         * synchronization object.
         *
         * @return {@code true} if owned
         */
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        /**
         * Returns an estimate of the number of threads waiting on
         * this condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        /**
         * Returns a collection containing those threads that may be
         * waiting on this Condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
         *
         * @return the collection of threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

    /**
     * Setup to support compareAndSet. We need to natively implement
     * this here: For the sake of permitting future enhancements, we
     * cannot explicitly subclass AtomicInteger, which would be
     * efficient and useful otherwise. So, as the lesser of evils, we
     * natively implement using hotspot intrinsics API. And while we
     * are at it, we do the same for other CASable fields (which could
     * otherwise be done with atomic field updaters).
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    static {
        try {
            stateOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset
                (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset
                (Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset
                (Node.class.getDeclaredField("next"));

        } catch (Exception ex) { throw new Error(ex); }
    }

    /**
     * CAS head field. Used only by enq.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS tail field. Used only by enq.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     */
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                                        expect, update);
    }

    /**
     * CAS next field of a node.
     */
    private static final boolean compareAndSetNext(Node node,
                                                   Node expect,
                                                   Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}
